Various metals or metal alloys, for example aluminium alloys, have up to now found numerous applications because of their advantageous properties, and in particular their mechanical properties, good thermal conductivity, light weight and low cost. Thus, for example, cooking utensils and appliances, anti-friction bearings, chassis or supports for equipment and various parts obtained by moulding are known. Copper, because of its excellent thermal conductivity, is also widely used for cooking appliances.
However, these metals or metal alloys have disadvantages linked to their low hardness, low wear-resistance and low resistance to corrosion.
Two essential problems arise with regard to cooking utensils. On the one hand, food has a tendency to stick to aluminium alloy surfaces during cooking. On the other hand, it is difficult to clean cooking devices having surfaces of insufficient hardness, (for example aluminium alloy grill pans). This type of device is easily cleaned by scraping. Such a procedure is, however, difficult to use for alloy surfaces of low hardness, because it leads to rapid deterioration in the condition of the surface.
Copper cooking utensils which traditionally have an internal coating of tin are also known. This coating, although it is particularly suitable for contact with food, nevertheless has the disadvantage of rapidly deteriorating due to its ductility.
Various solutions have been proposed to try to resolve these problems. One of the solutions consists in replacing aluminium alloys with other materials, for example steels, which may be stainless or provided with metal coatings. The advantages associated with good thermal conductivity are then lost. In addition, coatings have been proposed to prevent food sticking, for example Teflon coatings. However, such coatings resist scraping less well than the aluminium alloy substrate itself, and their thermal stability is relatively low.
Various attempts have been made to obtain improved aluminium alloys. Thus, European Patent 100287 describes a family of amorphous or microcrystalline alloys having an improved hardness, which are usable as reinforcing elements for other materials or for obtaining surface coatings which improve resistance to corrosion or wear. But a large number of the alloys described in this patent have a major disadvantage since they are subjected to a temperature greater than 200.degree. C. during use. In fact, they are not stable to heat, and during heat treatment, in particular the treatment to which they are subjected during deposition on a substrate, they change their structure: return to the microcrystalline state in the case of essentially amorphous alloys, increase in particle size in the case of essentially microcrystalline alloys which initially have a particle size of less than one micron. This change in crystalline or morphological structure leads to a change in the physical characteristics of the material which essentially affects its density. This results in the appearance of micro-cracks, and hence brittleness, which interfere with the mechanical stability of the deposits. See also, U.S. Pat. Nos. 4,595,429 and 4,710,246 by LeCaer.
Metal alloys have also been used as heat barriers.
Heat barriers are assemblies of one or more materials intended to limit heat transfer to or from equipment parts and components in many domestic or industrial appliances. There may be mentioned, for example, the use of heat barriers in heating or cooking appliances, smoothing irons where the hot part is attached to the body and where there is heat insulation; in automobiles, in a number of points such as the turbo-compressor, the muffler, the insulation of the passenger compartment, and the like; in aeronautics, for example in the rear part of compressors and jet engines.
Heat barriers are sometimes employed separately in the form of a screen, but very frequently they are directly associated with the source of heat or with the part to be protected, for reasons of mechanical strength. Thus, use is made of sheets of mica, ceramic plates and the like in domestic electrical appliances by adapting them by screwing or adhesive bonding, or of sheets of agglomerated glass wool which are supported by metal sheeting. A particularly advantageous process for attaching a heat barrier to a part, in particular to a metal part, consists in depositing onto a substrate the material constituting the barrier in the form of a layer of specified thickness using a thermal deposition technique such as, for example, plasma deposition.
It is very often recommended to combine the heat barrier with other materials which are also deposited as a layer by thermal deposition. These other materials may be intended to provide the barrier with protection against external attacks, for example mechanical impacts, a corrosive environment, and the like, or may be used as a primer for bonding to the substrate.
The material most frequently employed in aeronautics to form heat barriers is yttriated zirconia, which withstands very high temperatures. The deposition of zirconia is carried out by plasma deposition according to a conventional technique starting with the powdered material. The zirconia exhibits a very low thermal diffusivity (.alpha.=10.sup.-6 m.sup.2 /s). However, it has a relatively high specific mass .rho., and this is a disadvantage in the case of some applications; in addition, some of its mechanical properties, such as hardness and resistance to wear and to abrasion, are poor.
Other materials are employed as a heat barrier. There may be mentioned alumina, which has a specific mass which is lower than that of zirconia, and a diffusivity and a specific heat which are higher than that of zirconia, but whose mechanical properties are not satisfactory. There may also be mentioned stainless steels and some refractory steels, which offer thermal insulation properties, but which have a high specific mass.